Wiper devices (commonly known as "strippers") have been used on drilling rigs for about 20 years to strip drilling fluid from drill pipe as it is being pulled or "tripped" out of the well. In one embodiment, the device comprises a pair of horizontal, opposed rubber wiper pads, each forming a semi-circular opening in the leading edge, which are simultaneously advanced to engage opposite sides of the circular drill pipe. The pads are vertically staggered in that one is above the other so that they overlap in encircling the pipe. Each wiper pad is sandwiched and bolted between steel plates along the back and side edges of the pad. The plate assembly is referred to herein as the holder. Pneumatic, double-acting cylinders connected to the holders move them between a stripping position, wherein the pads are pressed against the drill pipe, and a retracted or open position. The stripped mud is caught in an underlying catch basin and recycled to the rig mud system.
U.S. Pat. No. 3,133,641, issued to the present inventor, discloses such a stripper in which the wiper pad assemblies "float". The main problem addressed by this patent was how to design the wiper pad assemblies so they could cope with the tendency of the drill string to sway laterally in the course of being pulled through the wiper pads. The drill string sways as it is pulled upwardly by the block and tackle of the rig. When being pulled, the drill string travels up through the annular rotary drive bushing of the rig rotary table. The bushing opening will commonly have an inside diameter of 105/8"--the drill pipe may have an outside diameter of 41/2". So there is a clearance on each side of the centered drill string of about 3". As a result, the drill string is free to sway laterally a few inches before it contacts the bushing and is stopped. The drill string is very heavy and its lateral movement can only be resisted by very heavy and strong structure. This means that the cylinders actuating the wiper pad assemblies cannot be anchored to a non-yielding structure. Otherwise the unyielding cylinder and wiper pad could be damaged by the oncoming swaying drill string. The answer provided by the patent was to mount the cylinders so that they bridged and were connected to the two wiper pad assemblies. The cylinders thus rode or floated with the wiper pads and were unaffected by the pipe sway.
In U.S. Pat. No. 4,457,366, also issued to the present inventor, a different approach was taken to providing floating capability. In this case, the cylinder actuating the wiper pad assembly was connected at its outer end to a vertical pivot pin anchored to a stationary support. The pad assembly and cylinder could thus pivot in a horizontal plane to accommodate lateral movement of the drill string. The head ends of the two actuating cylinders were connected by a line to allow one cylinder to dump air into the other, to enable the first cylinder to retract while the second extended, thereby accommodating to and fro movement of the drill string.
The known strippers just described have always been limited to use with land-based rigs. The stripper would be mounted in the rig sub-structure, beneath the rotary table bushing. It would be a permanent in-line structure through which the drill string would extend during drilling and tripping.
There has been a long standing need for strippers on large off-shore rigs. However, due to the extensive blow-out preventer equipment used with off-shore rigs, there has usually not been room beneath the rotary table to accommodate the conventional in-line stripper.
It is the objective of the present invention to provide a stripper which is adapted to be mounted and used either on the rig floor or below it, while preserving the ability of the wiper pad assemblies to float w ith the drill string.